Hello Rayi,
The following web links should answer most of your questions:
GREEN ROOFS
PASSIVE COOLING DESIGN
In regards to water collection, a slight slope on the roof and a gutter attached should be enough to direct clean water to a tank.
The thermal mass and foliage protects from direct heat, the evaporative effects should also cool, though it would be dependent on the surrounding environment e.g. typical tropical house design with large open verandahs, doors and louvered windows, water ponds, and foliage; rather than concrete, brick or stone.